Starting from a single stem cell, scientists have now produced a functional prostate gland. This marks only the second time researchers have generated a whole organ from a single stem cell, representing major advances for stem cell research and our understanding of prostate development, experts say.

Creating viable organs to transplant into ailing patients is a holy grail of stem cell research. The first success came in 2006 when researchers created mammary glands after discovering mammary stem cells in mice. Since then, researchers have been working furiously to coax stem cells into forming other organs. The prostate seemed a promising target because previous studies with mice revealed that it contained cells with the same protein markers as other known stem cells, suggesting that the necessary stem cells might be relatively easy to find.

Those studies suggested that the region of the prostate nearest the urethra might be rich in stem cells. So developmental biologist Wei-Qiang Gao and his colleagues at Genentech in South San Francisco, California, examined how these cells responded when they castrated six mice, which kills portions of the prostate, and injected them with testosterone to stimulate regrowth. Gao's team used a form of polymerase chain reaction analysis to scan the cells for signs of various proteins, including some previously found in stem cells and some known to play a role in prostate development.

They turned up a dark horse. Cells with a protein marker called CD117+, not previously associated with prostate stem cells, proliferated after castration and testosterone injection, the researchers found. When 97 cells with a particular set of markers, including CD117+, were implanted in mice kidneys, 14 of the transplanted cells generated almost full-sized prostates the researchers report online today in Nature. These stem-cell generated prostates secreted the same proteins as normal, functioning prostates.

Protein analysis of human prostates also detected CD117+ protein. If the protein turns out to be a marker for prostate stem cells in humans as well, Gao says it should prove useful for researchers investigating the hypothesis that prostate cancer arises from stem cells gone awry.

"This is a very significant finding," says Stephen Badylak, regenerative medicine specialist at the University of Pittsburgh in Pennsylvania. The study marks an important step toward the goal of organ regeneration, Badylak says, but more work is needed to ensure that cells in the regenerated organs know when to stop growing and what shape to take.